1. Field of the Invention
The present invention relates to a pneumatic tire having an air permeation preventive layer capable of reducing the weight of the tire, without impairing the retention of air pressure in the tire, and of reducing the cost of manufacturing a pneumatic tire by improving the air permeation preventive layer such as an inner liner layer.
2. Description of the Related Art
The reduction of fuel consumption is one of the major technical targets in the automobile industry. There have been increasingly stronger demands for reducing the weight of the pneumatic tires for this purpose. The inner surface of a pneumatic tire is provided with an inner liner layer composed of a low gas permeable rubber such as halogenated butyl rubber so as to enable the tire air pressure to be held constant. A halogenated butyl rubber, however, suffers from a large hysteresis loss. Thus, after vulcanization of the tire, there are waves formed in the inner surface rubber of the carcass layer and the inner liner layer in the space between the carcass cords and the inner liner rubber layer deforms along with the deformation of the carcass layer, and therefore, there is the problem that the rolling resistance increases. For this reason, the inner liner layer (e.g., halogenated butyl rubber) and inner surface rubber of the carcass layer are generally joined through a rubber sheet called a tie gum with a small hysteresis loss. Accordingly, in addition to the thickness of the inner liner layer of the halogenated butyl rubber, there is added the thickness of the tie gum and the layer, as a whole, becomes a thickness of over 1 mm (1000 .mu.m). As a result, this becomes one factor increasing the weight of the final tire product.
Various techniques have been proposed for using various materials in place of the low gas permeable rubber such as butyl rubber as the air permeation preventive layer such as an inner liner layer of the pneumatic tire. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-40207 proposes to provide an air permeation preventive layer at the inside of a tire by forming a thin film by laminating a low air permeation layer composed of a polyvinylidene chloride film or ethylene-vinyl alcohol copolymer film and a bonding layer composed of a polyolefin film, aliphatic polyamide film, or polyurethane film, laminating this thin film on the inner surface of a green tire composed of unvulcanized rubber so as that the bonding layer contacts the carcass layer, then vulcanizing the green tire. Use of such a multilayer film as the air permeation preventive layer enables the thickness of the air permeation preventive layer to be reduced from the conventional layer and enables the reduction of the weight of the tire, without impairing the retention of the air pressure. However, this air permeation preventive layer requires large equipment requiring several extrusion machines for making the multilayer film, and therefore, suffers from a large installation space, high manufacturing costs, and difficulty of controlling the molding temperature and the molding speed when forming the multilayer film, and therefore, the careful attention should be made for stable manufacture.
As explained above, the multilayer film for the air permeation preventive layer as described in Japanese Unexamined Patent Publication (Kokai) No. 6-40207 may be superior in performance, but has problems in manufacturing costs and working efficiency.